What Size Battery for a Trolling Motor?
Choosing the right battery for a trolling motor directly affects your time on the water. A battery that's too small leaves you stranded mid-lake, while an oversized battery adds unnecessary weight and cost. This guide walks through the engineering behind trolling motor battery sizing so you can match your battery to your motor, boat, and fishing style.
Quick Answer
For a 12V 30lb thrust trolling motor, use a single 12V 100Ah LiFePO4 or Group 31 AGM battery. For a 24V 55lb thrust motor, use two 12V 100Ah batteries in series. For a 36V 80lb thrust motor, use three 12V 100Ah batteries in series. LiFePO4 batteries provide the best runtime-to-weight ratio and are strongly recommended for fishing applications.
| Motor Voltage | Thrust | Batteries Needed | Recommended Size |
|---|---|---|---|
| 12V | 30–40 lbs | 1 × 12V | 100Ah LFP or Group 31 AGM |
| 24V | 50–70 lbs | 2 × 12V (series) | 100Ah LFP × 2 or Group 31 AGM × 2 |
| 36V | 70–100 lbs | 3 × 12V (series) | 100Ah LFP × 3 or Group 31 AGM × 3 |
How Trolling Motor Power Draw Works
A trolling motor's current draw is not constant — it varies dramatically with speed setting. At low speeds, the motor draws only a few amps. At full throttle, current draw increases exponentially. Understanding this relationship is critical for estimating real-world runtime, because most anglers don't run at full speed all day.
The relationship between speed and current draw is approximately: doubling the speed setting roughly triples the current draw. This means running at 50% speed doesn't use 50% of the power — it uses roughly 25–30% of full-speed power. The propeller load increases with the square of RPM, so higher speeds require disproportionately more energy.
Current Draw by Speed Setting
For a typical 12V 30lb thrust trolling motor, approximate current draw at various speed settings:
| Speed % | Current (A) | Power (W) | 100Ah LFP Runtime |
|---|---|---|---|
| 10% | 3A | 36W | ~26 hours |
| 25% | 6A | 72W | ~13 hours |
| 50% | 15A | 180W | ~5.3 hours |
| 75% | 25A | 300W | ~3.2 hours |
| 100% | 30A | 360W | ~2.7 hours |
Environmental Factors
Current draw increases significantly when fighting wind, current, or heavy vegetation. A motor running in calm water at 50% speed might draw 15A, but the same setting in a headwind or strong current could draw 20–25A. Plan your battery capacity for the conditions you typically fish in, not the best-case scenario.
Runtime Formula
For 24V and 36V systems, the total battery capacity in Ah is the capacity of a single battery (since batteries in series maintain the same Ah rating while voltage adds). A 24V system with two 100Ah batteries provides 100Ah at 24V — the same total energy as a single 200Ah 12V battery.
Worked Example 1: 12V Trolling Motor
Scenario: Half-day fishing trip (4 hours), 12V 30lb thrust motor, running at 50% speed.
Given:
- Motor: 12V, 30lb thrust, 15A at 50% speed
- Battery: 100Ah 12.8V LiFePO4
- DoD: 80%
- Runtime needed: 4 hours
Step 1: Usable capacity:
Step 2: Runtime at 50% speed:
Step 3: Check against trip requirement:
Result: The 100Ah LFP battery provides 5.3 hours of runtime at 50% speed — more than enough for a 4-hour half-day trip. You could increase speed to 75% and still have 3.2 hours of runtime.
Worked Example 2: 24V Trolling Motor
Scenario: Full-day tournament fishing (8 hours), 24V 55lb thrust motor, mixed speed usage.
Given:
- Motor: 24V, 55lb thrust
- Batteries: 2 × 12V 100Ah LiFePO4 (series = 24V 100Ah)
- Usage pattern: 4 hrs at 25%, 3 hrs at 50%, 1 hr at 75%
- 24V motor current: ~4A at 25%, ~10A at 50%, ~18A at 75%
Step 1: Energy consumed per speed segment:
50% speed: 10A × 3h = 30 Ah
75% speed: 18A × 1h = 18 Ah
Step 2: Total consumption:
Step 3: Usable capacity (80% DoD):
Step 4: Remaining capacity:
Result: Two 100Ah LFP batteries in series provide 80 Ah usable at 24V — enough for a full 8-hour tournament day with a 20% reserve. The 20% reserve accounts for unexpected conditions (wind, current, GPS anchoring).
Worked Example 3: 36V Trolling Motor
Scenario: Heavy-duty fishing with 36V 80lb thrust motor, full-day use (10 hours).
Given:
- Motor: 36V, 80lb thrust
- Batteries: 3 × 12V 100Ah LiFePO4 (series = 36V 100Ah)
- Average speed: 40% (conservative for heavy boat)
- 36V motor current at 40%: ~8A
Step 1: Usable capacity at 36V:
Step 2: Runtime:
Result: Three 100Ah LFP batteries provide 10 hours of runtime at 40% speed — a full day of heavy-duty fishing. At higher speeds (60%), runtime drops to approximately 6.7 hours, which is still sufficient for most tournament days.
Battery Chemistry Comparison for Trolling Motors
| Chemistry | Usable DoD | Weight (100Ah) | Cycle Life | Cost Range |
|---|---|---|---|---|
| LiFePO4 | 80–90% | 10–14 kg | 3,000+ | $800–$1,500 |
| AGM Lead-Acid | 50% | 28–32 kg | 300–500 | $150–$300 |
| Gel Lead-Acid | 50% | 28–32 kg | 400–600 | $200–$400 |
| Flooded Lead-Acid | 50% | 28–35 kg | 200–400 | $80–$150 |
Real-World Scenarios
Scenario 1: Small Boat, Casual Fishing
Jon boat with a 12V 30lb thrust motor. Fish for 3–4 hours in calm water at low speed (25%). A single 50Ah LiFePO4 battery provides approximately 6.5 hours of runtime — more than enough. At this usage level, a Group 24 AGM battery is also a viable budget option.
Scenario 2: Bass Boat, Tournament Fishing
Bass boat with a 24V 55lb thrust motor. Tournament day: 8 hours of mixed-speed fishing with GPS anchoring. Two 100Ah LiFePO4 batteries provide 80 Ah usable — sufficient for a full day with 20% reserve. The lightweight lithium batteries (24 kg total) vs lead-acid (60+ kg) improve boat handling and fuel economy.
Scenario 3: Kayak with 12V Motor
Kayak with a small 12V 20lb thrust motor for covering distance. Using a compact 50Ah LiFePO4 battery (6 kg). At low speed (30%), the motor draws 4A, providing 10 hours of runtime. The lightweight setup adds minimal weight to the kayak while providing all-day power.
Scenario 4: Large Bay Boat, Heavy Use
Large bay boat with a 36V 100lb thrust motor. Fighting strong tidal current all day. Three 100Ah LiFePO4 batteries in series. At 60% speed against current (12A draw), runtime is approximately 6.7 hours. For full-day protection, adding a fourth battery in parallel (creating a 36V 200Ah bank) doubles runtime to 13+ hours.
Scenario 5: Budget Setup with Lead-Acid
Angler on a budget using two Group 31 AGM batteries (100Ah each) for a 24V system. Usable capacity is 50Ah (50% DoD). At moderate speed (40%), the motor draws 8A, providing 6.25 hours of runtime. Weighs 60+ kg total — significantly heavier than lithium, but costs one-third the price.
Common Mistakes
Mixing Battery Types or Ages
Never mix batteries of different chemistries, capacities, or ages in a series or parallel string. The weakest battery limits the entire bank and degrades faster. Always use identical batteries and replace all batteries in a bank at the same time.
Using Starting Batteries
Car and boat starting batteries are designed for high-current bursts, not sustained deep cycling. Running a trolling motor from a starting battery will damage it within a few outings. Always use deep-cycle batteries for trolling motor applications.
Ignoring Speed vs Runtime Trade-off
Running at full speed seems efficient but consumes power exponentially faster. At 100% speed, a motor draws 6–9× more current than at 25% speed. Learning to run at lower speeds when covering distance dramatically extends battery life.
Undersizing for GPS Anchoring
GPS anchoring (Spot-Lock) constantly adjusts motor output to hold position, often running at higher average speeds than manual trolling. In windy or current-heavy conditions, Spot-Lock can increase power consumption by 30–50%. Size your batteries with this in mind.
Calculate Your Exact Battery Requirement
Use the Marine Battery Sizing Calculator to size a complete trolling motor battery system based on your motor specs and fishing patterns.
Open Marine Battery CalculatorRelated Tool
Calculate exact runtime for your trolling motor and battery configuration with the Runtime Calculator.
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Read Guide →Frequently Asked Questions
What size battery do I need for a trolling motor?
A trolling motor battery must deliver both capacity (runtime) and current (thrust). For a 12V 30lb thrust motor, a single 12V 100Ah LiFePO4 or Group 27/31 AGM battery is sufficient. For 24V or 36V systems (55–80lb thrust), you need 2 or 3 batteries wired in series to achieve the required voltage, with each battery sized for your desired runtime.
How long will a 100Ah battery last on a trolling motor?
At low speed (30% thrust), a 12V 30lb trolling motor draws approximately 5–8 amps, providing 12–20 hours of runtime on a 100Ah battery. At medium speed (50% thrust), draw increases to 15–20 amps, providing 5–7 hours. At full speed (100% thrust), the motor draws 30+ amps, providing only 3–3.5 hours.
Can I use a car battery for a trolling motor?
A car starter battery is not recommended. Starter batteries are designed for high-current bursts, not sustained deep cycling. Running a trolling motor will drain and damage a starter battery quickly. Use a dedicated deep-cycle battery — AGM, gel, or LiFePO4 — designed for sustained discharge.
What is the best battery for a trolling motor?
LiFePO4 (LFP) batteries are the best choice for trolling motors. They provide 80–90% usable capacity, weigh 50–70% less than lead-acid, charge faster, and last 3,000+ cycles. For budget-conscious anglers, AGM lead-acid batteries offer good performance at lower upfront cost. Gel cells are also suitable but less common.
How do I wire batteries for a 24V trolling motor?
Connect two 12V batteries in series: positive of battery 1 to negative of battery 2. The remaining positive and negative terminals provide 24V. Both batteries must be identical (same capacity, chemistry, age, and brand). Never mix different batteries in a series string.
How do I wire batteries for a 36V trolling motor?
Connect three 12V batteries in series: positive of battery 1 to negative of battery 2, then positive of battery 2 to negative of battery 3. The remaining positive and negative terminals provide 36V. All three batteries must be identical. Use a 36V onboard charger for convenient charging.
Does a trolling motor need a deep cycle battery?
Yes. Trolling motors draw sustained current for hours, which is exactly what deep-cycle batteries are designed for. Starting batteries will fail prematurely under this type of discharge. Always use a deep-cycle battery (AGM, gel, or lithium) for trolling motor applications.
How do I maximize trolling motor battery life?
Run the motor at the lowest speed necessary for your conditions. Higher speeds consume exponentially more power. Use GPS anchoring (Spot-Lock) instead of manual positioning to reduce runtime. Charge batteries fully after each use. Store batteries at full charge in a cool, dry location. Maintain proper water levels in flooded lead-acid batteries.
What is the difference between 12V, 24V, and 36V trolling motors?
Higher voltage systems provide more thrust with lower current draw. A 36V 80lb motor draws less current per pound of thrust than a 12V 30lb motor, allowing thinner wiring and more efficient operation. 12V suits small boats and light use. 24V is the most popular for bass boats. 36V is for heavy-duty applications requiring maximum thrust.
How many batteries do I need for a 24V trolling motor?
A 24V trolling motor requires exactly two 12V batteries wired in series. The total capacity (Ah) of each battery determines runtime. For a full day of fishing (6–8 hours), use two 100Ah LiFePO4 batteries or two Group 31 AGM batteries.